GeV Gamma Rays from Supernova Remnants Interacting with Molecular Clouds

TL;DR

This paper reports on Fermi LAT observations of middle-aged supernova remnants interacting with molecular clouds, revealing bright GeV gamma-ray emission that informs shock acceleration and cosmic ray origins.

Contribution

It provides new observational evidence linking gamma-ray emission to molecular cloud interactions in supernova remnants, supporting models of cosmic ray acceleration.

Findings

Middle-aged SNRs are brighter in GeV than TeV gamma rays.

Spectral steepening is observed in the Fermi-LAT band.

Most gamma-ray bright SNRs interact with molecular clouds.

Abstract

We report the current status of the observations of supernova remnants (SNRs) with the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope, focusing on middle-aged SNRs that appear to be interacting with molecular clouds. Observations with the Fermi LAT in an energy range from 0.2 to ~100 GeV have unveiled the presence of luminous GeV gamma-ray emission in middle-aged SNRs, providing a new insight into the shock-acceleration theory and the origin of galactic cosmic rays. The middle-aged SNRs detected by the Fermi LAT are generally much brighter in GeV than in TeV in terms of energy flux, which emphasizes the importance of the GeV observations. Spectral steepening in the Fermi-LAT band is commonly found for the GeV-luminous SNRs. Remarkably, most (if not all) of them are known to be interacting with molecular clouds, and they are also the strong sources of synchrotron radio emission. We discuss possible scenarios to explain the enhanced GeV gamma-ray emission in the cloud-interacting SNRs. Particular emphasis is placed on a scenario in which shock-accelerated cosmic-rays are adiabatically compressed and energized as result of radiative cooling behind the cloud shock.